WIP: Add Store instruction to MIR for variable assignments

- Added Store instruction generation in build_assignment()
- This partially addresses the VM infinite loop issue
- However, the loop still uses old values (%0) instead of updated values
- Need to implement proper SSA phi nodes for loop variables

The root cause: MIR generation doesn't properly track variable updates
in loops. Current SSA implementation lacks phi nodes.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

local_tests/test_vm_simple_loop.nyash

@@ -0,0 +1,12 @@
+// 超シンプルなVMループテスト
+local i = 1
+print("Before loop: i = " + i)
+
+// 3回だけループ
+loop(i <= 3) {
+    print("In loop: i = " + i)
+    i = i + 1
+    print("After increment: i = " + i)
+}
+
+print("After loop: i = " + i)